The present invention relates to a hydraulic control system of an automatic transmission for an industrial vehicle such as a fork lift truck, a shovel loader or a bulldozer and more particularly to an arrangement to cool the oil in such hydraulic control system, the transmission herein referred to including a hydrokinetic torque converter and multiple-plate clutches.
Industrial vehicles perform frequent forward and backward movements at very low speeds and are subjected to frequent switching between forward and backward movements due to the nature of their usage so that temperature of the oil within a hydrokinetic torque converter is apt to rise so that the mean temperature of the oil circulating through the hydraulic control system also rises because the heated oil returns from the torque converter to a oil reservoir or oil tank.
This undesirable temperature rise of the oil lowers the viscosity of the oil so that poor lubrication performance results, sealing effectiveness decreases and sealing rubbers tend to change their characteristics at a fast rate. If an increase in temperature of the oil is excessive, the oil is subject to degradation and sludge is deposited.
In a conventional automatic transmission for an industrial vehicle an oil cooler is provided and is fluidly connected to an exhaust passage for exhausting oil from a torque converter to cool the oil from the torque converter, wherein the oil after being cooled by the oil cooler is drained to an oil tank.
The oil cooler takes the form of a heat exchanger disposed in a lower tank of an engine coolant cooling radiator so that the cooling capacity is inevitably fixed. Thus with the oil cooler the temperature of the lubricant can not be maintained within a predetermined temperature range providing appropriate viscosity if there is a drastic change in environment conditions or in its usage. Hence allowable change in environment conditions and in usage of an industrial vehicle is quite limited by its oil cooler capacity.